Cherry-pick #21930 to 7.x: [libbeat] Fix potential deadlock in the disk queue + add more unit tests

libbeat/publisher/queue/diskqueue/core_loop.go

@@ -169,8 +169,16 @@ func (dq *diskQueue) handleReaderLoopResponse(response readerLoopResponse) {
 		// A segment in the writing list can't be finished writing,
 		// so we don't check the endOffset.
 		segment = dq.segments.writing[0]
+		if response.err != nil {
+			// Errors reading a writing segment are awkward since we can't discard
+			// them until the writer loop is done with them. Instead we just seek
+			// to the end of the current data region. If we're lucky this lets us
+			// skip the intervening errors; if not, the segment will be cleaned up
+			// after the writer loop is done with it.
+			dq.segments.nextReadOffset = segment.endOffset
+		}
 	}
-	segment.framesRead = uint64(dq.segments.nextReadFrameID - segment.firstFrameID)
+	segment.framesRead += response.frameCount
 
 	// If there was an error, report it.
 	if response.err != nil {
@@ -346,14 +354,29 @@ func (dq *diskQueue) maybeReadPending() {
 		// A read request is already pending
 		return
 	}
+	// Check if the next reading segment has already been completely read. (This
+	// can happen if it was being written and read simultaneously.) In this case
+	// we should move it to the acking list and proceed to the next segment.
+	if len(dq.segments.reading) > 0 &&
+		dq.segments.nextReadOffset >= dq.segments.reading[0].endOffset {
+		dq.segments.acking = append(dq.segments.acking, dq.segments.reading[0])
+		dq.segments.reading = dq.segments.reading[1:]
+		dq.segments.nextReadOffset = 0
+	}
+	// Get the next available segment from the reading or writing lists.
 	segment := dq.segments.readingSegment()
 	if segment == nil ||
 		dq.segments.nextReadOffset >= segmentOffset(segment.endOffset) {
 		// Nothing to read
 		return
 	}
 	if dq.segments.nextReadOffset == 0 {
-		// If we're reading the beginning of this segment, assign its firstFrameID.
+		// If we're reading the beginning of this segment, assign its firstFrameID
+		// so we can recognize its acked frames later.
+		// The first segment we read might not have its initial nextReadOffset
+		// set to 0 if the segment was already partially read on a previous run.
+		// However that can only happen when nextReadFrameID == 0, so we don't
+		// need to do anything in that case.
 		segment.firstFrameID = dq.segments.nextReadFrameID
 	}
 	request := readerLoopRequest{